In a typical internal combustion engine the engine crankshaft is mounted for rotation in bearings mounted on the cylinder block. The bearings are typically split with one half of the bearing supported in the cylinder block and the other half of the bearing supported in a bearing cap mounted to the cylinder block. The bearing caps retain and locate the crankshaft, or bearings for a crankshaft shaft which in turn retain and locate the shaft, relative to a cylinder block. The journal bearings on the crankshaft run against the two half shell bearings which are fitted to the main bearing cap and semi-circular surfaces on internal supporting walls of the block. For vibration free, low friction and quiet running, the roundness of the bore produced by the main bearing cap and the bulkhead is very important. This roundness is achieved by a machining operation called line boring. The main bearing caps are bolted to the internal supporting walls and then all the circular bearing surfaces are machined in a single operation. This ensures the two half rounds formed by the main bearing cap and the bearing block form as near to a perfect circle as possible and that all the circular bearings are coaxial. The bearing surfaces may then be honed to a fine finish to achieve the extremely fine tolerances needed for quiet running and efficient engine performance.
However, to install the crankshaft, it is necessary to remove the main bearing caps from the engine block. After the crankshaft is put in place, it is necessary to reposition the main bearing caps so that they are replaced in the identical position they occupied during the line boring operation. Any deviation from that original position produces an out-of-round condition that, in turn, leads to vibration, noise and possibly stiff, high friction crankshaft operation and rapid wear. There are a number of conventional methods for relocating and attaching the main bearing caps to bulkheads when installing the crankshaft. In one such method the main bearing cap has a very precisely machined dimension across the transverse axis (that is normal to the rotational axis of the crankshaft journal) of the main bearing cap across the foot of the bearing cap adjacent the bearing split line. This dimension is known as a lock width or snap width. A pair of cooperating opposing surfaces are precision machined on pads on the engine block support wall to produce a controlled interference fit with the cap when the main bearing cap is refitted after crankshaft installation.
The location in the fore and aft direction (i.e., in the direction of the rotational axis of the crankshaft journal) may be controlled by the bearing cap bolts or the use of hollow dowels pressed into counter-based holes in the support wall. The dowels then locate in precisely machined counter-bores in the corresponding main bearing cap.
The interference fit between the main bearings caps and pads on the engine block cause some stresses to both the cap and the cylinder block. The machined pad surfaces and the machined clamping surface for the mounting cap on the support wall are substantially normal to each other and meet at a relatively sharp line of intersection. The lock width pads are subject to high compressive forces during the fitting of the main bearing cap and during operation of an engine, loads generated by combustion will also be transferred to the lock width pads.
The present invention provides a cylinder block in which the stresses caused by the lock width for the main bearing caps are reduced increasing the durability of the engine.
According to a first aspect of the present invention there is provided a cylinder block for an internal combustion engine, the cylinder block in use supporting a rotary crankshaft, the block having pairs of opposed lock width surfaces formed on the block for forming an interference fit with respective co-operating lock width surfaces on the crankshaft bearing caps which in use are secured to respective bearing cap support surfaces also formed on the block, wherein the lock width surfaces are spaced apart from the respective bearing cap support surfaces.
The lock width surfaces spaced from each respective bearing cap support surface such that a portion of the lock width surface in use aligns with the apex of the bearing cavity in the cap and preferably the mid height of the lock surface is at a distance from the bearing cap support surface substantially equal to the radius of the crankshaft bearing.
The lock width surfaces on the block may also be displaced relative to the end of the bearing cap mounting surface (that is radial with respect the axis of rotation of the crankshaft).
The cylinder block may be cast from any suitable material, such as cast iron or aluminium, and is particularly advantageous for materials which have a high notch sensitivity such as compacted graphite iron (CGI).
According to a second aspect of the invention there is also provided an internal combustion engine having a cylinder block according to a first aspect of the invention with a crankshaft rotationally mounted in bearings on the cylinder block, at least some of said bearings comprise two half bearings with one half of each bearing being on the block and the other half thereof being on a respective bearing cap mounted to the respective bearing cap support surface, each bearing cap having spaced apart lock-width surfaces thereon which form an interference fit with the respective co-operating lock width surfaces on the block.
The total interference fit is preferably in the order of 10–200 microns per bearing cap.
Each bearing cap comprises a body having limbs passing on each side of a semi-circular half bearing cavity and has mounting surfaces on the ends of each limb that sit against the support surface on the block, the outer end portions of each limb (that is outwardly from the crankshaft) adjacent the mounting surfaces having clearance with respect to the block.
Preferably each bearing cap is secured on the support surface bolts vertically (that is normal) to the support surface, and by cross-bolts (that is bolts parallel to the support surface) that engage with opposite outer sides of the bearing cap.
Conveniently, each cross bolt engages with a raised pad on the outer side of each limb.
Preferably the lock width surfaces are formed on the two cross bolt pads.
In the preferred arrangement, the crankshaft runs against the two half shell bearings which are fitted to the main bearing cap and semi-circular surfaces on internal supporting walls of the block.
The invention also comprises a motor vehicle having an engine according to the second aspect of the present invention.